shoddy - Python/C API investigation

shoddy.c

#include <Python.h>

typedef struct {
  PyListObject list;
  int state;
} Shoddy;


static PyObject *
Shoddy_increment(Shoddy *self, PyObject *unused)
{
  self->state++;
  return PyLong_FromLong(self->state);
}


static PyMethodDef Shoddy_methods[] = {
  {"increment", (PyCFunction)Shoddy_increment, METH_NOARGS,
   PyDoc_STR("increment state counter")},
  {NULL,NULL},
};

static int
Shoddy_init(Shoddy *self, PyObject *args, PyObject *kwds)
{
  if (PyList_Type.tp_init((PyObject *)self, args, kwds) < 0)
    return -1;
  self->state = 0;
  return 0;
}

static void
Shoddy_dealloc(Shoddy *self)
{
  PyListObject *lo;
  Py_ssize_t i;

  lo = (PyListObject *)self;

  if (lo->ob_item != NULL){
    i = Py_SIZE(lo);
    while(--i >=0) {
      Py_XDECREF(lo->ob_item[i]);
    }
    PyMem_FREE(lo->ob_item);
  }
  Py_TYPE(self)->tp_free((PyObject *)self);

}

static PyTypeObject ShoddyType = {
  PyObject_HEAD_INIT(NULL)
  "shoddy.Shoddy",            /* tp_name */
  sizeof(Shoddy),             /* tp_basicsize */
  0,                          /* tp_itemsize */
  (destructor)Shoddy_dealloc, /* tp_dealloc */
  0,                          /* tp_print */
  0,                          /* tp_getattr */
  0,                          /* tp_setattr */
  0,                          /* tp_reserved */
  0,                          /* tp_repr */
  0,                          /* tp_as_number */
  0,                          /* tp_as_sequence */
  0,                          /* tp_as_mapping */
  0,                          /* tp_hash */
  0,                          /* tp_call */
  0,                          /* tp_str */
  0,                          /* tp_getattro */
  0,                          /* tp_setattro */
  0,                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT |
  Py_TPFLAGS_BASETYPE,        /* tp_flags */
  0,                          /* tp_doc */
  0,                          /* tp_traverse */
  0,                          /* tp_clear */
  0,                          /* tp_richcompare */
  0,                          /* tp_weaklistoffset */
  0,                          /* tp_iter */
  0,                          /* tp_iternext */
  Shoddy_methods,             /* tp_methods */
  0,                          /* tp_members */
  0,                          /* tp_getset */
  0,                          /* tp_base */
  0,                          /* tp_dict */
  0,                          /* tp_descr_get */
  0,                          /* tp_descr_set */
  0,                          /* tp_dictoffset */
  (initproc)Shoddy_init,      /* tp_init */
  0,                          /* tp_alloc */
  0,                          /* tp_new */
};

static PyObject *
make_a_shoddy(PyObject *self, PyObject *args, PyObject *kw)
{
  Shoddy *shd;
  PyObject *tup, *item;
  PyListObject *lo;
  Py_ssize_t i, size;
  size_t nbytes;

  tup = Py_BuildValue("(iiiii)", 1, 2, 3, 4, 5);
  size = PyTuple_GET_SIZE(tup);
  printf("tup built\n");
  shd = PyObject_NewVar(Shoddy, &ShoddyType, size);
  if (shd == NULL){
    printf("allocation failed\n");
    Py_DECREF(tup);
  }
  printf("shd allocated\n");

  // cast shoddy to a list and initialize
  lo = (PyListObject *) shd;
  nbytes = size * sizeof(PyObject *);
  lo->ob_item = (PyObject **) PyMem_MALLOC(nbytes);
  if (lo->ob_item == NULL) {
            Py_DECREF(lo);
            return PyErr_NoMemory();
        }
  memset(lo->ob_item, 0, nbytes);
  lo->allocated = size;
  printf("list style allocation successful\n");
  printf("Py_SIZE(list) : %ld\n", Py_SIZE(lo));
  printf("Py_SIZE(shoddy) : %ld\n", Py_SIZE(shd));
  for(i = 0; i < size; i++){
    item = PyTuple_GET_ITEM(tup,i);
    Py_INCREF(item);
    PyList_SET_ITEM((PyObject *)shd, i, item);
  }
  Py_DECREF(tup);
  shd->state = 0;
  return (PyObject *) shd;
}

static PyMethodDef shoddymethods[] = {
  {"make_a_shoddy", (PyCFunction) make_a_shoddy, 
   METH_VARARGS | METH_KEYWORDS, "make a shoddy"},
  {NULL, NULL} /* Sentinel */
};

static PyModuleDef shoddymodule = {
  PyModuleDef_HEAD_INIT,
  "shoddy",
  "Shoddy module",
  -1,
  shoddymethods, 
  NULL, NULL, NULL, NULL
};

PyMODINIT_FUNC
PyInit_shoddy(void)
{
  PyObject *m;

  ShoddyType.tp_base = &PyList_Type;
  if (PyType_Ready(&ShoddyType) < 0)
    return NULL;

  m = PyModule_Create(&shoddymodule);
  if (m == NULL)
    return NULL;

  Py_INCREF(&ShoddyType);
  PyModule_AddObject(m, "Shoddy", (PyObject *) &ShoddyType);
  return m;
}